In hydrothermal acid regeneration, ferric oxides, in particular Fe2O3, precipitate from a 75% aqueous FeCl3 solution at a temperature of approximately 170° C. and are filtered out.
The hydrothermal acid regeneration in metal processing, in particular after pickling of metal strips, is concerned with the regeneration of hydrochloric acid contaminated with FeCl2. To do this, the following method steps are carried out.                (1) The hydrochloric acid containing FeCl2 is concentrated.        (2) FeCl2 is oxidized with O2 at 150° C. and 7 bar to form FeCl3:2FeCl2+2HCl+½O2→2FeCl3+H2O        (3) Fe2O3 precipitates during hydrolysis at 170° C. and atmospheric pressure. Reusable 18%-30% hydrochloric acid is obtained:2FeCl3+3H2O→FeO3↓+6HCl↑        
In industrial production, Fe2O3 is separated from the solution by a belt filter. A belt filter is very useful in industrial production for separating solid substances from liquids. The liquid to be filtered is conveyed onto the filter fleece web. The solid particles are retained on the filter fleece. The cleaned liquid flows into the storage tank and can be discharged from there for reuse or disposal. The residue remaining on the fleece forms a filter cake. If the density of the filter cake obstructs an optimal outflow of the liquid, the filter cake together with the fleece is transported automatically into a waste tank. The entire process is carried out continuously and fully automatically without interrupting the flow of liquid.
It has been found that the known filter cloths are not suitable to be used in a vacuum belt filter.